It is well known that lactic acid or hydroxy-2-propanoic acid which is an α-hydroxy carboxylic acid can be produced by fermentation. Further processes for obtaining lactic acid are known to those skilled in the art, via chemical conversions of petrochemical reagents such as the hydrolysis of lactonitrile obtained from acetaldehyde, chlorination and hydrolysis of propionic acid or via propene nitration.
It is also known that lactic acid is found in two diastereoisomeric forms, the L(+) form and the D(−) form, and encounters novel applications every day, from the conventional use as a food preservative to novel developments such as the synthesis of solvents, pesticides, herbicides, biodegradable polymers, etc.
However, due to the increasing stringency of the required quality criteria and the need to achieve production costs compatible with the commodities market, it is vital to be able to reduce energy costs while maintaining a quality level meeting the strictest requirements. Moreover, controlling, or reducing, the energy consumption of industrial processes is of particular interest in the current context of environmental pressure and the restriction of fossil energy resources.
It is also known that the purity of a lactic acid is, among other things, evaluated by measuring the colour at ambient temperature (APHA scale in Hazen units) and by a thermal stability test consisting of measuring the colour (APHA scale in Hazen units) of the substance after heating at reflux at a temperature of 200 degrees Celsius (° C.) for two hours. A lactic acid solution is generally considered to be heat-stable if the colour after cooling to ambient temperature does not exceed 50 Hazen.
However, for some specific applications such as for example for the production of polylactic acid, lactic acid should have a very high purity and a very low thermal stability index, generally less than 50 Hazen, or in some cases less than 30 Hazen. It consists of a “polymer” grade if it enables the production of a polylactic acid having a molecular weight greater than 100,000 Dalton as specified in the patent (EP 1953 234 A1).
Furthermore, the prior art describes in detail that the industrial purification of a heat-stable grade lactic acid from a fermentation juice rich in lactic acid can be carried out using various technologies generally including common steps:                Clarifying the fermentation must: (centrifugation, flocculation/filtration, microfiltration, etc.)        Removing the ions (electrodialysis, ion exchange resins, liquid/liquid extraction, etc.)        Removing the colour and other impurities (membrane filtration, activated carbon, etc.)        Concentrating/distilling the lactic acid: these steps should be combined to obtain a high yield.        
These purification methods are described for example in the patents (U.S. Pat. Nos. 6,489,508; 5,681,728; 7,244,596). Using these techniques for purifying a lactic acid having a concentration greater than 85%, a specific colour greater than 500 Hazen and derived from fermentation, at the present time, it is necessary to use at least one distillation step to produce a heat-stable grade lactic acid.
The disadvantage of these types of methods is the amount of energy required and the complex equipment required.
It is also known that lactic acid in concentrated solution can be crystallised (H. Borsook, H. M. Huffman, Y-P. Liu, J. Biol. Chem. 102, 449-460 (1933), L. B. Lockwood, D. E. Yoder, M. Zienty, Ann N.Y. Acad. Sci. 119, 854 (1965), Holten C. H., “Lactic acid: Properties and chemistry of lactic acid and derivatives”, 20-22, Verlag Chemie, 1971).
The methods according to the prior art describing the production of lactic acid crystals in a crystallisation stage do not enable the production of heat-stable grade lactic acid crystals unless either a lactic acid of relatively high quality (colour <500 Hazen) is used as the initial lactic acid, or organic solvents are used. All these constraints will have a non-negligible impact on production costs. Furthermore, none of these methods mentions the size of the crystals obtained.
In the patent WO 0222545, a method for purifying lactic acid is also described, but including, before the crystallisation step, an extraction step in organic solvent, instead of distillation. It is known to those skilled in the art that a small percentage of organic extractant is found in the aqueous phase from this extraction, requiring an additional purification step to remove this solvent residue.
However, in some cases, the impure fermentation juice may be purified without using organic solvent.
The crystallisation of lactic acid is also described in the patent WO 0056693, but requires starting with a much purer aqueous lactic acid solution (the colour of the lactic acid solution not exceeding 83 Hazen).
The literature also contains methods for purifying lactic acid, particularly as described in the patent WO 0222544, including one or a plurality of crystallisation steps associated with a distillation step.
The purity of crystals is generally associated with the specific surface area by mass thereof.
The specific surface area by mass (SSM) of crystals is the area developed by the crystals per unit of weight. The specific surface area by mass (SSM) is used to compare the dimensional characteristics of crystals of a suspension or a powder with those of another suspension or powder. This specific surface area can be measured by means of optical imaging on the basis of a volume (Vm) and the mean surface area (Sm) of hundreds of crystals (by measuring the length of the face thereof) and the density of the crystals (Dc):SSM=Sm/(Vm*Dc).
Indeed, after the liquid/solid separation following crystallisation, the quantity of residual parent solution per unit of mass of crystals is proportional to the surface area thereof. The impurities being essentially found in the parent solution, the smaller the surface area developed by the crystals per unit of mass and the lower impurity content of the mass of crystals. For example, this is the case of beet and cane sugar.
It is also known that liquid/solid separation is also facilitated with large crystals.
There is a need for a method for producing, by crystallisation, heat-stable grade lactic acid at least cost from an impure aqueous solution having a colour greater than 500 Hazen working at low temperatures, preferably not exceeding 30° C. and more preferentially between 4 and 26° C.
During crystallisation, said step may particularly be influenced by two factors, nucleation and supersaturation.
There are two types of nucleation, primary nucleation and secondary nucleation.
In the case of primary nucleation, micro-organisms appear in a medium not yet containing any crystals from the precipitating phase. If the micro-organisms are formed in the volume of the solution, the nucleation is referred to as primary homogeneous. If, on the other hand, they are formed on the walls of the crystallisers, on the stirrers or on solid particles floating in the solution, the nucleation is referred to as primary heterogeneous.
In the case of secondary nucleation, micro-organisms appear in a medium wherein crystals from the precipitating phase already exist. If the solution contains crystals, they may collide with each other, hit the walls, the stirrer, other solid particles and thus release microscopic particles. These particles can then grow. Secondary nucleation may also occur following a sudden rise in supersaturation.
The nucleation temperature (Tn) mentioned in the present patent application consists of the secondary nucleation temperature.
In the metastability zone, the supersaturation (S) in gallons per liter (g/l) at a given temperature can be defined as the difference between the concentration in the solution (C) (g/l) and the concentration at saturation (C*) (g/l): S=C−C*.
The degree of supersaturation (Dsc) of a solution at a given concentration “c” is: Dsc=(Tsc−T)/(Tsc−Tnc).
Where Tsc is the solubilisation temperature, T is the operating temperature, and Tnc is the secondary nucleation temperature.
The very structure of lactic acid bearing both a hydroxyl function and a carboxylic acid group gives rise to condensation reactions generating lactoyllactic, dilactoyllactic, trilactoyllactic, . . . (n-lactoyllactic) units also referred to as lactic acid oligomers. These condensation or oligomerisation reactions tends towards equilibrium but are all the more likely if the starting aqueous solution concentration and temperature are high (Holten C. H., “Lactic acid: Properties and chemistry of lactic acid and derivatives”, Verlag Chemie, 1971).
The monomer content in relation to the total lactic acid concentration (monomer and oligomer) can be estimated using the following equation provided that the total acidity is less than 105%:Relative monomer content=(TA%−(TA %−FA%)*2)/TA %,where: TA=total acidity determined by acid-base titration after saponification and expressed as lactic acid monomer; and FA=free acidity determined by direct acid-base titration and expressed as lactic acid monomer.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the disclosure and such exemplifications are not to be construed as limiting the scope of the disclosure in any manner.